Adding Initial PKCE Auth Flow Support (#542)

* Added base funtionality for PKCE Authorization - i538

* fixed a mistake with the auth code

* fixed more misunderstandings. fixed grant_access_token to now call authorization if needed

* added comments and references to code verifier and code challenge

* removed debug print statement

* updated unit tests for new PKCE flow

* cleaned up username issues - added doc strings to class

* fixed import issue, added user endpoint tests

* forgot to commit this file

* linted

* clarified comment

* no longer generates code verifier or challenge in constructor, only when needed

* fixed flake8 complaints, added forgotten unit tests

* fixed linting with unit tests

* anotha one

* added python3.5 support

* linting

* added to CHANGELOG

* removed as_dict option from get_access_token()

Co-authored-by: tomCLANCC <26153156+tomCLANCC@users.noreply.github.com>

CHANGELOG.md

@@ -16,6 +16,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
  - Proper replacements for all deprecated playlist endpoints
  (See https://developer.spotify.com/community/news/2018/06/12/changes-to-playlist-uris/ and below)
 - Allow for OAuth 2.0 authorization by instructing the user to open the URL in a browser instead of opening the browser.
+- Support for the PKCE Auth Flow
 
 ### Deprecated